Conventionally, a three-panel, projection-type image displaying apparatus (color projector) employs a strong light source such as a metal halide lamp to emit white light. The white light is separated into red (R), green (G), and blue (B) primary color beams to irradiate reflective spatial light modulators, respectively. The modulators are driven by R, G, and B video signals to modulate the R, G, and B beams and reflect the modulated beams, which are combined and projected.
FIG. 1 is a perspective view showing a conventional image displaying apparatus. This is a three-panel, projection-type image displaying apparatus employing reflective spatial light modulators, disclosed in Japanese Patent Application Laid-Open Publication No. H10-197949. The image displaying apparatus has upper and lower optical systems arranged in upper and lower layers, respectively.
In the upper optical system, a light source 11 emits a beam of light, which is passed through a collimator lens 20 and first and second integrators 21a and 21b. The beam is reflected by a cold mirror 22 so that the beam is deflected by 90 degrees. The beam is passed through a third integrator 21c and an infrared cut filter 23 and is injected into a front face of a three-color-separating cross dichroic prism 24. The first to third integrators 21a, 21b, and 21c form an integrator 21. The prism 24 separates the incident beam into red (R), green (G), and blue (B) primary color beams, which emanate from the side and back faces of the prism 24, respectively, into three directions. The three primary-color beams are injected into upper polarizing beam splitter prisms 12r, 12g, and 12b, respectively. The polarizing reflective faces of the prisms 12r, 12g, and 12b reflect each an s-polarized component toward the lower optical system.
Output beams from the upper polarizing beam splitter prisms 12r, 12g, and 12b are passed through convex lenses 13r, 13g, and 13b and polarizers 14r, 14g, and 14b, respectively, and are injected into lower polarizing beam splitter prisms 15r, 15g, and 15b, respectively.
In the lower optical system, the beams injected into the lower polarizing beam splitter prisms 15r, 15g, and 15b are reflected by polarizing reflective faces thereof, are passed through wave plates 16r, 16g, and 16b, and are injected into reflective spatial light modulators 17r, 17g, and 17b, respectively. If skewed light components are injected into the polarizing reflective face of any one of the prisms 15r, 15g, and 15b, a beam transmitted through and emitted from the prism will have a polarized state that is not linear. Accordingly, the wave plates 16r, 16g, and 16b correct the phase characteristics of beams transmitted through the prisms 15r, 15g, and 15b and provide linearly polarized beams. At the same time, the wave plates 16r, 16g, and 16b correct pre-tilted states of liquid crystals in the modulators 17r, 17g, and 17b. 
The reflective spatial light modulators 17r, 17g, and 17b modulate the incident beams in response to video signals and reflect the modulated beams, which return to the lower polarizing beam splitter prisms 15r, 15g, and 15b. Only the modulated components are transmitted through the polarizing reflective faces of the prisms 15r, 15g, and 15b. 
The beams transmitted through the polarizing reflective faces of the lower polarizing beam splitter prisms 15r, 15g, and 15b enter the side and back faces of a three-color-combining cross dichroic prism 25, respectively. The color beams injected into the prism 25 are combined into one, which emanates from the front face of the prism 25 and enters a projection lens 18.
The beam injected into the projection lens 18 is projected therefrom onto a screen (not shown) to display an image.
The polarizing beam splitter prisms used in the image displaying apparatus according to the conventional art mentioned above are each an optical element having a polarizing reflective face that, ideally, reflects 100% of an s-polarized component and transmits 100% of a p-polarized component.
FIG. 2A is a graph showing the wavelength dependence of transmittance of a p-polarized component in a polarizing beam splitter prism, and FIG. 2B is a view showing an incident angle β between the optical axis L0 of a polarizing beam splitter prism 30 and an incident beam L1.
The polarizing beam splitter prism can realize various characteristics depending on a film structure formed on a polarizing reflective face. In practice, it is impossible to provide ideal characteristics. The prism realizes a relatively high extinction ratio for an s-polarized component but a low extinction ratio for an s-polarized component. More precisely, the polarizing reflective face reflects more than 99%, nearly 100% of a linear s-polarized incident beam, and therefore, transmits substantially no part of the s-polarized beam. On the other hand, for a p-polarized incident beam, the prism transmits about 90% of the beam at the maximum and about 20% thereof at some incident angles and wavelengths, as shown in FIG. 2A. The remaining ten to several tens of percents of the p-polarized beam is reflected as leakage in the same direction as a reflected s-polarized beam. This is a disadvantage of the conventional art.
To overcome the disadvantage, the image displaying apparatus according to the conventional art employs the upper polarizing beam splitter prism serving as a pre-polarizer in front of the lower polarizing beam splitter prism serving as a main polarizer. The pre-polarizer prevents an unnecessary polarized component from mixing with an incident beam to the reflective spatial light modulator and improves the purity of an s-polarized component. Namely, the pre-polarizer prevents a p-polarized component from mixing with an incident beam to the lower polarizing beam splitter prism, so that only an s-polarized component may be reflected by the polarizing reflective face of the lower polarizing beam splitter prism toward the reflective spatial light modulator. When the reflective spatial light modulator reflects a modulated beam toward the lower polarizing beam splitter prism, the polarizing reflective face of the prism reflects nearly 100% of an unnecessary s-polarized component and transmits only a p-polarized component, thereby improving a contrast ratio of an image to be displayed.
In this way, the image displaying apparatus according to the conventional art needs the polarizing beam splitter prism serving as a pre-polarizer, which increases the size of the optical system of the image displaying apparatus.
In recent years, there are demands for projection-type image displaying apparatuses capable of displaying bright images. To increase the brightness of a displayed image, the f-value of an optical system-of an image displaying apparatus must be decreased. Reducing the f-value of an optical system increases an angle between a beam and an optical axis, to increase skewed components in a beam injected into a polarizing beam splitter prism. The skewed components deteriorate the polarization and separation characteristics of the prism, thereby decreasing the contrast ratio of displayed images.
This problem is caused because the polarizing beam splitter prism has a disadvantage of fluctuating the polarization and separation characteristics thereof depending on beam's incident angles. The polarizing beam splitter prism is a rectangular prism having an oblique face coated with several tens of layers of dielectric films made by vapor deposition. Two or more kinds of dielectric materials having different refractive indexes are alternately laid one upon another on the oblique face. At each interface between the layers, refracted light and reflected light cause a phase interference to provide functions of reflecting an s-polarized component and transmitting a p-polarized component. A phase interference relationship between the refracted light and the reflected light at each interface differs depending on an incident angle of light. Namely, the polarizing and separating characteristics of a polarizing beam splitter prism change according to an incident angle of light.
FIG. 2A is a graph showing the wavelength dependence of transmittance of a p-polarized component in a visible wavelength zone in a polarizing beam splitter prism with an incident angle β between an incident beam and a transmission face (incident face) serving as a parameter.
In FIG. 2A, a curve “a” represents an incident angle β of 0 degrees, “b” −6 degrees, “c” −15 degrees, “d” +6 degrees, and “e” +15 degrees. The transmission face of the polarizing beam splitter prism is orthogonal to an optical axis, and the incident angle β is equal to an angle between an incident beam to the prism and the optical axis. As shown in FIG. 2A, the wavelength dependence of transmittance of a p-polarized component is relatively constant when the incident angle β is within ±6 degrees. When the incident angle β exceeds the range of ±6 degrees, the transmittance of a p-polarized component becomes greatly dependent on wavelengths and the transmittance itself decreases.
A deterioration in the polarizing and separating characteristics of the polarizing beam splitter prism leads to a decrease in the contrast ratio of an image displayed.
The image displaying apparatus employing the polarizing beam splitter prism sometimes causes shading (unevenness) on a projection screen. This is caused when the polarizing face of the prism partially rotates within the prism. To prevent this, double refraction (distortion) in the glass material of the prism must be minimized. Namely, the prism must be made of a material having a small photoelastic constant.
Employing such material makes the manufacturing of the polarizing beam splitter prism more difficult and increases the cost thereof. In addition, such material involves a large specific weight to increase the total weight of the optical system of the image displaying apparatus. If a large reflective spatial light modulator is employed, the polarizing beam splitter prism must be enlarged accordingly, to greatly increase the total weight of the optical system.
The image displaying apparatus according to the conventional art employs the dichroic prism in the upper optical system that carries out color separation. Namely, the upper optical system for color separation has substantially the same size as the lower optical system for color composition. In this arrangement, a beam from the light source gradually converges toward the reflective spatial light modulators, to cause an eclipse particularly in the upper optical system for color separation. The eclipse deteriorates beams qualitatively and quantitatively. The image displaying apparatus according to the conventional art, therefore, demonstrates a poor light usage ratio and is unable to display sufficiently bright images.